Urinary flow rates, urinary urate concentrations, and urinary urate excretion rates were compared in 3 groups of anesthetized (sodium pentobarbital) femal Sprague-Dawley rats before and for 3 hr after i.v. injections of saline, cephaloridine (250 mg/kg) or probenecid (250 mg/kg). Cephaloridine approximately doubled the rate of urine flow during the first hour after injection. Although the urinary urate concentration did not change significantly, the urinary urate excretion rate increased from 0.77 ug/min to 1.4 ug/min (6 rats). This "uricosuric effect" of cephaloridine may be the result of the diuresis. The doubling of the urate excretion rate was much less than the 5 to 10-fold increases which occurred in rats injected with probenecid.